The present invention relates generally to a method and apparatus for continuously dispensing precise amounts of a liquid and more particularly to a method and apparatus for maintaining precise concentrations of a dye within a liquid solution or paper slurry.
It is recognized in the paper industry that it is extremely important to maintain a very constant color intensity within a paper run. There are two primary issues involved in maintaining constant color intensity: (1) diluting a concentrated dye to a production concentration and (2) mixing appropriate amounts of the production concentration to the slurry to maintain a constant color intensity in the final product. Over the years it was the practice in paper mills to prepare large batches of diluted dye for each production run. Each batch was held in one or more large tanks, which were then used to supply dye during the course of the paper production run. It is readily apparent that this batch system was expensive to construct and maintain and that it led to much waste of diluted dye because an excessive amount of diluted dye had to be prepared to ensure the supply would not run out before the run was completed. Excess diluted dye could only be disposed rather than saved for another production run. Also, the holding tanks and related plumbing required regular cleaning between batches, generating substantial costs in both materials and personnel. Furthermore, either several sets of tanks had to be maintained in order to change colors during a run or a succeeding color had to wait until the tanks and plumbing were cleaned and a new batch of dye was prepared. In any event, the entire batch system was quite cumbersome and costly to operate.
Various types of pumps have been used to supply dye solutions from storage tanks to slurry tanks in paper mills. Precision ground gear pumps and plunger pumps have been used with some success, but they suffer from several inadequacies. In particular, because the dye comes into direct contact with the interior surfaces of the pumps, substantial cleaning of the pump is required when a color is changed. Also, the gear type pumps lose precision over time because the dyes aggressively attack the pump materials and the plunger type pumps suffer from check valve failures (which prevent dye delivery). It has been found that peristaltic pumps overcome many of these problems. In particular, a peristaltic pump urges liquid through an elastic tube by sequentially pinching the tube between a rotating roller and a fixed surface. There is no direct contact between the dye solution and the pump mechanism and therefore there is no need to clean the pump mechanism when a dye is changed. The elastic tubes are either stored for re-use with a similar dye or discarded because of their relatively modest cost. Also, because the dye solution does not contact the pump mechanism, it cannot attack the pump materials and thereby affect the effective volume of the dye solution being delivered.
Although the peristaltic pumps have proven beneficial, a single peristaltic pump is most precise in its operation when it is sized for the desired volume of dye being dispensed. That is to say, a small peristaltic pump obviously cannot handle large volumes well nor can a large peristaltic pump make precise adjustments in flow at low flow rates. Therefore it has still been necessary to replace pumping equipment when substantially different volumes of dye are being added to a slurry.
Accordingly, it is an object of the present invention to provide a method and apparatus for preparing a diluted solution on a continuous basis while minimizing the possibility of running out of solution while a concentrate supply tank is replaced, for example.
It is also an object of the present invention to provide a method and apparatus for dispensing precise amounts of a liquid over a wide variety of flow rates using a single set of peristaltic pumps.
These and other objects of the present invention will become apparent upon consideration of the drawing referred to hereinafter and a complete description thereof.
In accordance with the present invention, a method and apparatus is provided for continuously dispensing a precisely controlled amount of liquid from a holding tank into a receiving tank. An elastic coarse tube and an elastic fine tube each permit flow of fluid between the holding tank and the receiving tank. The coarse tube is sized to allow a substantially greater flow rate of the liquid than the fine tube is sized to allow. The flow rates of liquid through the coarse tube and the fine tube are independently controlled by a coarse peristaltic pump and a fine peristaltic pump, respectively. The flow rate of liquid through the coarse tube is set at a level less than the desired total flow rate of liquid into the receiving tank and maintained at a generally constant level. The flow rate of liquid through the fine tube is regularly adjusted to precisely maintain the desired total flow rate of liquid into the receiving tank. In the event that the desired total flow rate of liquid into the receiving tank cannot be achieved through adjustment of the flow rate through the fine tube, the flow rate through the coarse tube is incrementally reset and again maintained at a generally constant level while the flow rate through the fine tube is again regularly adjusted to maintain precise control of the total flow rate. (The pumps are reranged for control.)
The holding tank may be continuously supplied with liquid through an elastic diluent (e.g. water) tube from a diluent source and an elastic concentrate tube allowing fluid flow from a concentrate source. The flow rate of diluent through the diluent tube is controlled by a diluent peristaltic pump. The flow rate of concentrate through the concentrate tube is independently controlled by a concentrate peristaltic pump.
An electronic controller may be used to constantly monitor the total flow rate into the receiving tank, constantly monitor the volume of the liquid in the holding tank and independently adjust the speeds of the peristaltic pumps and hence the total throughput.